a) Field of the Invention
The present invention relates to a lens system for use with photographic cameras.
b) Description of the Prior Art
The standard lens systems currently used for single-lens reflex cameras are generally designed as the so-called Gauss type which is composed of about six lens elements arranged nearly symmetrically with regard to an aperture stop. Due to this arrangement of the lens element nearly symmetrical with regard to the aperture stop, the Gauss type lens system requires a relatively large number of lens elements and a high manufacturing cost though the offaxial aberrations are corrected favorably therein.
As lens systems consisting of small numbers of lens elements, there are known the triplet type and Tessar type lens systems. However, it is difficult to design these types of lens systems so as to have high aperture ratios and aberrations corrected sufficiently favorably.
Further, there are known lens systems each of which is composed of four lens elements arranged completely symmetrically with regard to an aperture stop, as exemplified by the lens system disclosed by Japanese Patent Kokai Publication No. 51-8932. However, this lens system has a low aperture ratio of F/8 and is not practical for use as a standard lens system for general photographic cameras.
Moreover, a symmetric type lens system which consists of four lens elements including aspherical lens elements is illustrated in FIG. 57, on page 31, of the Camera Review magazine.
This lens system also has a low aperture ratio of F/6.3 and is not suited for use as a standard lens system for photographic cameras. In addition, this literature clarifies no numerical data for the lens system and constitutes no reference for lens design.
As is understood from the foregoing description, each of the conventional Gauss type lens systems used as the standard lens system for photographic cameras comprises lens elements in the large number of about six, or when the lens system is composed of lens elements in a small number of about four, it has a low aperture ratio and aberrations insufficiently corrected.